Higher operating temperatures for gas turbine engines are continuously sought in order to increase efficiency. However, as operating temperatures increase, the high temperature durability of the components within the engine must correspondingly increase.
Significant advances in high temperature capabilities have been achieved through the formulation of nickel- and cobalt-based superalloys. For example, some gas turbine engine components may be made of high strength directionally solidified or single crystal nickel-based superalloys. These components are cast with specific external features to do useful work with the core engine flow and often contain internal cooling details and through-holes to provide external film cooling to reduce airfoil temperatures.
When exposed to the demanding conditions of gas turbine engine operation, particularly in the turbine section, the base alloy alone may be susceptible to damage, such as oxidation and corrosion attack, and may not retain adequate mechanical properties. Accordingly, the base alloys are often protected with various types of coating systems depending upon the engine part and operating environment.
Thermal barrier coatings are a key element in current and future gas turbine engine designs expected to operate at high temperatures, which produce high thermal barrier coating surface temperatures. One desired system for a hot high temperature engine part includes a strain-tolerant thermal barrier ceramic layer deposited onto a bond coating, which exhibits good oxidation/corrosion resistance.
Under service conditions, thermal barrier coated engine parts can also be susceptible to various modes of damage, including erosion, oxidation, and attack from environmental contaminants. At temperatures of engine operation, adherence of these environmental contaminants on the hot thermal barrier coated surface can cause damage to the thermal barrier coating. Environmental contaminants may form certain compositions, which are liquid at the surface temperatures of thermal barrier coatings.
Chemical and mechanical interactions occur between the contaminant compositions and the thermal barrier coatings. Molten contaminant compositions can dissolve the thermal barrier coating or can infiltrate its pores and openings, initiating and propagating cracks causing delamination and loss of thermal barrier coating material.
Some environmental contaminant compositions that deposit on thermal barrier coating surfaces contain oxides mainly of calcium, magnesium, aluminum, silicon, and mixtures thereof with possible minor oxide additions of titanium, iron, nickel, chromium and mixtures thereof. These oxides combine to form contaminant compositions comprising calcium-magnesium-aluminum-silicon-oxide systems (CaO—MgO—Al2O3—SiO2), herein referred to as CMAS. Damage to thermal barrier coatings occurs when the molten CMAS infiltrates the thermal barrier coating. After infiltration and upon cooling, the molten CMAS, or other molten contaminant composition, solidifies. The stress build up in the thermal barrier coating may cause cracking and/or spallation of the coating material and loss of the thermal protection that it provides to the underlying part. Alternately or in addition, the CMAS can react chemically with the TBC to accelerate thermal sintering or dissolve stabilizing components such as Y2O3 resulting in damage to the TBC coating.
U.S. Pat. No. 5,660,885 discloses sacrificial oxide protective coatings to be applied over a thermal barrier coating. In particular, this patent discloses sacrificial oxide protective coatings of alumina, magnesia, chromia, calcia, scandia, calcium zirconate, silica, spinels such as magnesium aluminum oxide, and mixtures thereof. U.S. Pat. Nos. 5,851,678, 5,871,820, 5,914,189 and 6,627,323 disclose various methods for covering a thermal barrier coating with barrier materials. In particular, these patents disclose impermeable barrier coatings of metal oxides, metal carbides, metal nitrides, metal suicides, precious metals, platinum group metals and mixtures thereof, which inhibits liquid contaminant compositions from infiltrating into or reacting with the thermal barrier coating at the operating temperature of the thermal barrier coating.
Despite the above advances, there is an ongoing need to reduce or prevent the damage to the thermal barrier coatings caused by the reaction or infiltration of molten contaminant compositions at the operating temperature of the engine. Embodiments of the invention fulfill this need and others.